Tube expanders of various constructions are conventionally used to expand the walls of hollow-shaped workpieces. The expansion may be free in which deformation of the external surface is not limited, or by containment in which deformation is limited by an external structure.
One form of tube expander is a rotary swage in which a plurality of rollers are mounted about a central mandrel in such a way as to cause the rollers to expand radially outwardly against the inner wall of the tube coincident with rotation of the mandrel and rollers about a central axis. The passing of successive rollers over a portion of the inner wall surface, coupled with the resultant spring-back or resistance of the wall to permanent deformation, acts as a series of blows or strikes to incrementally deform the tube into an expanded-diameter shape.
Rotary swage tube expanders have proven to be effective on thin-wall tubes. However, it has been found that under the greater forces needed to deform thicker-walled tubing the rollers have a tendency to pick-up material from the inner wall surface and to produce a condition known as spalling. In addition, a greater number of incrementally provided blows by rotary passes of the rollers over the wall surface are needed. Such additional working has a tendency to produce work hardening of the deformed material which can result in the subsequent failure of the workpiece.
Another form of tube expander is a simple solid punch having a shape substantially equal to the desired dimension of the expanded workpiece. This form of expander is generally drawn or pushed through the workpiece and the wall is axially extruded to the desired shape. Solid punch-tube expanders are again effective if the deformation forces are of a relatively low magnitude. Fixturing of the workpiece to resist the axial force of the entering punch must be provided. The fixturing must also prevent the tube itself from buckling under high columnar loading. The deformation force available is also limited by the physical and material limitations of the workpiece itself.
Yet another form of tube expander is found in a tool having a plurality of forming members mounted about a cone-shaped mandrel. As the mandrel carrying the forming members is forced into the workpiece, the mandrel cone radially displaces the forming members mounted thereon into forcible contact with the wall inner surface. If the cone is urged into the tube via a series of blows, such as from a hammer, cold-working of the wall and the aforementioned problems attendant thereto may again result. Deforming the tube by a single blow produces radial extrusion, which if not closely controlled may result in thinning and cracking in the deformed zone.
Another serious problem inherent with this latter form of prior art tube expander is attributable to separation of the forming elements as they are radially outwardly displaced by the cone-shaped mandrel. Separation of the segments produces a non-continuous peripheral contact force acting on the wall inner surface. Such non-uniform contact forces may form ridges or other surface anomalies deleterious to the effective future use of the product.
What is desired is a tube expander providing continuous circumferential contact with the workpiece throughout the annular deformation zone. Also, preferably, such a tool would be readily and easily insertable into a proximal end of the workpiece, and would accomplish the deformation by controllably extruding the desired portion in a single operation.